[Development] Changes to continuous integration in qtdeclarative/dev

[Frederik Gladhorn]

Hi all,

I'd like to give everyone a heads up (of hopefully good news). We plan to 
switch over the dev branch of qtdeclarative to a new continuous integration 
system in the beginning of next week. We currently aim for Monday if all goes 
well.

There will be a more comprehensive blog post about the new system in the near 
future, but it seems only fair to give a short intro here. One goal is to take 
advantage of the modularization which has been done on a per module level in 
Qt, but is so far only partially reflected in the continuous integration 
system.

In order to cut out long waiting times for git checkouts, we will create 
source tar balls centrally using git archive which are then downloaded by the 
build agents.
Agents are clones of virtual machine snapshots, so they should always be in a 
good shape, runs should be much more reproducible (apart from system clock and 
network dependencies). Once a module is built, we save the build artifacts 
(the build library binaries etc) and then reuse the build artifacts for the 
modules depending on it. This way we do not rebuild qtbase for qtdeclarative, 
unless it changed since the last run. There is still a lot of potential for 
optimization at this stage, but first we want the system to be reliable.

We should be covering all platforms/operating systems/configurations that we 
are currently running in the CI, but it would be great if everyone working on 
the module could have an eye on the test logs, in case we did miss something. 
One feature we will no longer support are reverse dependencies. After many 
long discussions we concluded that the best way forward is to have fast 
turnaround times (potentially reverting patches quicker if they are found to 
block other modules, until they have been fixed) combined with frequent builds 
of all modules (at least daily, probably more often).

If you run into trouble or see suspicious behavior, talk to Simon, Lars and 
me, we'll all be keeping an eye on things running smoothly.

Cheers,
Frederik